I want to endorse u/professorgerm for his post below, I was quick to talk about the 'hill to die on' and I think he's posed a very interesting question on what everyone's "red lines" are.
I would like to pose a different related question, which is how one might assess the difference between:
Alex
Alex genuinely supports some broader set of beliefs B and a sub-belief X, and, at the same time, genuinely believes that position X is not one to fight over for any number of strategic reasons. Perhaps he thinks X is not liable to get much support because it's not catchy, perhaps he's keeping his powder dry for a bigger fight over Y, perhaps it's timing (there are a number of very good political ideas from both the left & right that died due to sailing into contingent political winds), perhaps there is a unpopular personality associated to X at the moment. For any of these reasons, he thinks fighting over X is likely to lose and to do so in a way that weakens the broader set of beliefs rather than strengthening it. When he says "this is not the hill to die on", he has the best interest of the broader belief at heart, even if he's made an incorrect strategic decision.
Bart
Bart is like Alex that he genuinely supports B, but he thinks X is out in the bailey and believes it might be closer to the opponent's motte than to his. From an ideological point of view, he doesn't believe X is essential to the motte or that it is the best place to cleave reality from an intellectual perspective. When he says "this is not the hill to die on", what he's saying is that there is a cohesive set of ideological positions that he would take for which this is (perhaps just barely) outside it.
Carl
Carl supports B, but not very strongly. He's probably liable to surrender all or most of it if pressed. He said "this isn't the hill to die on" not out of a strategic but because the valley behind the hill just isn't that important to him. He'd keep it if he could, but it's not his priority. This is probably a lot of normies. When he says "this isn't the hill to die on", he means he's rather grill and drink beer than fight any of this stuff.
Don
Don doesn't really support B, he's probably closer to the other camp, but he also values some amount of give-and-take. He perceives (rightly or wrongly) that X is closer to his motte than to yours and, in general, is willing to concede some matters closer to yours. When he says "this isn't the hill to die on", he's saying that he thinks this hill is an easy win for his side and would rather keep more amicable relations.
[ It may be that Don does or does not deserve such considerations personally. It might also be that he's a decent person but that his side isn't more broadly and so they don't deserve it. ]
Elmer
Elmer is a true believer on the other side, same as Alex. Everything he says is strategically designed to gain more ground, you should assume this is his intent.
Missing Taxonomies
This is not meant to be totally exhaustive, but if you see others do let me know
The Question
As a proponent of B, how would you distinguish between these 5 (or more) cases? What indicia would you use to assess either the strategic (Alex) or ideological (Bart) arguments made when someone says "this is not the hill to die on". How would you distinguish them from Carl, Done or Elmer?